The present invention relates to an optical 3D digitizer, a system based on the digitizer and a corresponding method for digitizing an object, for example a full human being. The present invention has numerous applications, for example for computer-assisted 3D vision, human body digitizing, computer animation, computer graphics, electronic gaming, 3D electronic archiving, 3D web, reverse engineering and medical 3D imaging.
3D digitizing, particularly non-contact optical 3D digitizing techniques, became commercially available during the recent years. Most of these techniques are based on the principle of optical triangulation. Despite the fact that passive optical triangulation (stereo vision) has been studied and used for many years for photogrametic measurements, the active optical triangulation technique (particularly laser scanning technique) gained popularity because of its robustness and simplicity to process obtained data using a computer. Most of the systems based on the active optical triangulation principle were developed for industrial applications, such as robotic assembly, robot guidance, industrial inspection, reverse engineering, etc.
As an example of such technique, a laser beam or a laser stripe is projected on a 3D surface of an object, scattering the laser beam or laser stripe on the surface. It is measured using a photo-electronic device. A signal can be generated to indicate the position (usually the depth) of the measured point. In most cases, the basic measurements are either a point or a section profile. A mechanical or optical scanning device is usually used to provide a frame of 3D measurements. For industrial applications, mechanical scanning can be accomplished by the mechanism on which the digitizing device is mounted, such as a robot or a conveyer. The scanning process consists of a sequential data acquisition process and takes a relatively longer time to scan a surface. During the scanning, the object should be kept immobilized; this is a major problem when scanning a live being. Different techniques, such as the projection of multiple stripes, laser line scanning during one video frame and high speed scanning, have been developed. These approaches are either too expensive to realize, or their sampling rate is still too low compared to 2D digital imaging.
A laser beam is a monochromatic light source. One single monochromatic laser beam can not provide full color information of the measured surface. On the other hand, a number of today""s 3D applications including computer animation, electronic games, 3D web, 3D archiving and 3D medical imaging require information on color texture which contributes to most of the visual effects. In order to measure the color texture of a surface, a 3D digitizing system based on a laser scanning principle must use multiple laser sources (blue, green and red lasers) or use a second camera to get color data. The first solution is very difficult to be implemented and is also very expensive. The second can suffer from problems of misalignment between 3D geometric data and color texture data because they are not captured from the same angle of the view.
When digitizing a full human body, the required ratio between height and width of the measured zone should be 2 to 3 over 1. A system based on laser scanning is more flexible to provide a desired ratio, but its acquisition speed is too slow. All other systems using frame capturing of a CCD camera are limited by the geometric form of the sensor. Most of commercially available CCD sensors have an aspect ratio equal either to 4 or 3 over 1. If such a sensor is used to cover a human body possibly higher than 2 meters, the resulting lateral resolution would be very low. At the same time, many of the pixels are not useful for a measurement.
Known in the art are U.S. Pat. No. 3,619,033 (McMahon); U.S. Pat. No. 3,705,261 (Langley); U.S. Pat. No. 4,622,462 (Eaton et al.); U.S. Pat. No. 4,702,257 (Moriyama et al.); U.S. Pat. No. 4,775,235 (Hecker et al.); U.S. Pat. No. 4,957,369 (Antonsson); U.S. Pat. No. 5,037,207 (Tomei et al.); U.S. Pat. No. 5,198,877 (Schulz); U.S. Pat. No. 5,276,546 (Palm et al.); U.S. Pat. No. 5,313,265 (Hayes et al.); U.S. Pat. No. 5,315,512 (Roth); U.S. Pat. No. 5,377,011 (Koch); U.S. Pat. No. 5,386,124 (Yasuda et al.); U.S. Pat. No. 5,418,608 (Caimi et al.); U.S. Pat. No. 5,432,703 (Clynch et al.); U.S. Pat. No. 5,440,496 (Andersson et al.); U.S. Pat. No. 5,465,284 (Karellas); U.S. Pat. No. 5,559,712 (Kihara et al.); U.S. Pat. No. 5,630,034 (Oikawa et al.); U.S. Pat. No. 5,668,894 (Hamano et al.); U.S. Pat. No. 5,747,822 (Sinclair et al.); U.S. Pat. No. 5,804,830 (Shafir); U.S. Pat. No. 5,815,275 (Svetkoff et al.); U.S. Pat. No. 5,842,473 (Fenster et al.); U.S. Pat. No. 5,850,290 (Horiguchi et al.); 5,851,115 (Carlsson et al.); U.S. Pat. No. 5,864,640 (Miramonti et al.); U.S. Pat. No. Re. 34,566 (Ledley); and U.S. Pat. No. Re. 35,816 (Schulz). The above-mentioned patent documents provide a global idea of the state of the art.
An object of the invention is to address the various weaknesses in the existing optical 3D digitizers, and to provide a reliable solution for a cost-effective system.
Another object of the invention is to provide a digitizer, a system based on such a digitizer and a digitizing method which are relatively much faster than the presently available digitizers, digitizing systems and methods.
According to the present invention, there is provided an optical 3D digitizer for digitizing an object, comprising a white light source adapted to produce white light, a projection lens optically coupled to the white light source and arranged to project the white light toward the object whereby the object has a fully illuminated side, a grating device optically coupled between the white light source and the projection lens for selectively producing a fringe pattern in the light projected by the projection lens, and first and second cameras positioned aside from the projection lens and aligned in angled directions with respect to each other so that the cameras have complementary fields of view directed on the illuminated side of the object and partially overlapping with each other over a depth of measurement of the object, the cameras having respective video outputs to produce video signals representing complementary images of the object with a common image portion as a result of the fields of view being partially overlapping.
According to the present invention, there is also provided an optical 3D digitizer system for digitizing an object, comprising the aforesaid optical 3D digitizer provided with a control circuit connected to the white light source and the grating device, and a computer including a frame grabber having inputs for receiving the video signals from the cameras, the computer having a communication link with the control circuit of the digitizer.
According to the present invention, there is also provided an optical 3D digitizing method for digitizing an object, comprising the steps of: projecting white light toward the object using a single white light source, whereby the object has a fully illuminated side; selectively producing a fringe pattern in the light projected on the object; and capturing complementary images of the object illuminated by the white light using first and second cameras positioned aside from the white light source and aligned in angled directions with respect to each other so that the cameras have complementary fields of view directed on the illuminated side of the object and partially overlapping with each other over a depth of measurement of the object, the cameras having respective video outputs to produce video signals representing the complementary images of the object with a common image portion as a result of the fields of view being partially overlapping.
The following provides a non-restrictive summary of embodiments and certain features of the invention which are described with more details hereinafter.
The optical 3D digitizer according to the invention can be used in particular for digitization of a full human body. The cameras can be embodied by two standard color (color version) or monochromatic (B/W version) cameras. The white light source can be embodied by a projector. The two cameras are set in a way that over all the depth of the measurement, their captured images are always overlapped. The two images from two cameras can be merged to form one single image. Incidentally, the aspect ratio of combined image varies between 2 to 3 over 1. One or a few combined images will provide 3D measurement of one view of a human body. The acquisition time of one view requires a fraction of a second using commercially available standard cameras and frame grabbers. The cameras used for the measurement of 3D geometry provide also the capturing of color or gray scale texture, depending on the cameras. Since the same image pixel of the camera measures the 3D geometry and texture data of a point on a 3D surface, the texturing mapping on top of 3D geometry is automatically ensured by the nature of this data acquisition. The 3D digitizer based on the present invention does not need two image sensors to separately measure 3D geometry and texture and avoids the problem of misalignment occurring with laser scanning systems.
Different approaches can be taken for the 3D coordinate measurements. A first one uses one video frame containing a projected fringe pattern and a second one requires a few video images which also contain a projected fringe pattern. An image processing based on the analysis of the mechanical interference pattern provides the 3D coordinate data for each image pixel. An encoding process is applied for conversion of the measurement in computer units to real physical parameters. A set of encoding points is generated by the projected pattern and the absolute positions of these points can be determined once they are measured by the cameras. In fact, a function describing the absolute positions of the encoding points and their measured position on the photo sensitive area of the cameras can be defined experimentally after a digitizer is assembled. Each camera should preferably capture at least one encoding point. The encoding point ensures first the conversion of the measurement in computer unit to real physical parameters for the whole surface and indicates the geometric relation of the 3D images measured by each of the two cameras. A fine tuning procedure using the 3D data on the overlapped surface gives final adjustment to the positions of the two images. Both 3D geometric data and texture data acquired by the two cameras can be merged to form one single 3D model with one single texture image.
A defocusing optical element which removes fringe patterns from the image and provides a uniform illumination can be used. In order to keep light intensities similar for two images grabbed using two separate cameras, a procedure for light intensity adjustment is implemented, using the average light intensity measured on the overlapped area of the two images. The data obtained from each camera should be calibrated in a common coordinate for both cameras. Both 3D geometric data and texture data acquired by the two cameras are merged to form one single 3D model with one single texture image. A complete model of a human body can be thereby created using a single or multiple optical full human body 3D digitizers according to the invention.
An image of 640xc3x97480 pixels can be grabbed using one standard NTSC camera. Although there is an overlapped area between the two video images captured by the two cameras, the final merged image can still keep at least 1100 to 1200xc3x97480 pixels. These image pixels are distributed over a field of view corresponding to the form of a human body. So, there are no wasted image pixels. In addition, this number of sampling over the field of view ensures a very reasonable lateral resolution for final 3D measurement. The number of image pixels captured by each camera is limited either by industrial standards (for example, NTSC or PAL) or by the manufacturing cost of the camera. This limitation does not apply to the projected pattern. In fact, the only limit for a projected pattern (for example, a film) is the optical resolution of the film and projection optics. It is not uncommon to obtain a resolution of 50 to 100 lines per mm on a pattern to be projected which may have a size of 35 mmxc3x9725 mm. So one projected pattern can easily provide the necessary image information for the area covered by the two cameras. The major advantage of using one single projector instead of two is to avoid the cross-talking results from simultaneous captured images and two fringe patterns if two projectors were used.
The necessary acquisition time of the system according to the present invention is much shorter than most of existing techniques based on laser scanning principles and many more data points can be measured on a person who does not need special training to be kept immobilized for several seconds.
Different approaches can be taken to create a complete model of a human body using one or multiple optical full human body 3D digitizers according to the present invention. When one digitizer is used to capture multiple views of a human body, one has to rotate the person to be digitized or rotate the digitizer around the person so that each necessary view can be measured. Each sequential measurement of the surfaces preferably overlaps with the others. It is not necessary to know the exact position of each acquisition. The texture and geometric data on the overlapped area can be used to ensure the registration of each partial model. In order to reduce the total acquisition time, it is possible to use a number of 3D digitizers mounted in a fixed space. Four to six digitizers are usually needed to minimize uncovered surfaces. When this approach is used, the procedure for the registration becomes more simple because the positions of each view are well known.